单选题 Mr. Black was young and able. The boss liked him. Last month he was sent to China on business. Before he went back from China, he decided to boy something for Tony, the son of his boss. At first he didn't know what to buy for the boy. Suddenly he remembered that the buy liked birds very much, so he went to the bird market. He was glad to find that there were so many kinds of birds there. He bought four beautiful birds, then he told the boy about it on the telephone.
As soon as Mr. Black returned to London by air, he went to Tony with the four birds. But on his way to the boss's home, one of them flew out of the cage (笼子) and there were only three birds left in it. Then Mr. Black bought a local (当地的) bird to take the place of the missing Chinese one.
Tony of course was very happy when he saw the birds. But soon he found one of them was different from the other three. He asked, "It's a local bird, isn't it, Mr. Black?"
"Yes, it is." Mr. Black answered, "It's an interpreter ( 译员 )./